Security cards are widely used for various applications such as identification purposes (ID cards) and financial transfers (credit cards). Such cards typically consist of a laminated structure consisting of various paper or plastic laminates and layers wherein some of them may carry alphanumeric data and a picture of the card holder. So called ‘smart cards’ can also store digital information by including an electronic chip in the card body.
A principal objective of such security cards is that they cannot be easily modified or reproduced in such a way that the modification or reproduction is difficult to distinguish from the original.
Two techniques frequently used for preparing security documents are laser marking and laser engraving. In literature, laser engraving is often incorrectly used for laser marking. In laser marking, a colour change is observed by local heating of material, while in laser engraving material is removed by laser ablation.
US 2005001419 (DIGIMARK) discloses a colour laser engraving method and a security document including an opaque surface layer and one or more coloured sub-layers. A laser provides openings in the surface layer to expose the colour of the sub-layer thereby creating colour images and text.
WO 2009/140083 (3M) discloses methods to generate a colour image in a multilayer article containing at least one thermally activatable layer coated from a composition including a non-linear light to heat converter, a leuco dye, a thermal acid generator and a solvent. A colour image is formed in the colour forming layer upon activation with non-linear light beam radiation (300-1500 nm).
U.S. Pat. No. 7,158,145 (ORGA SYSTEMS) discloses a three-wavelength system (440, 532 and 660 nm) for applying coloured information to a document by means of wavelength-selective bleaching of chromophoric particles in a layer close to the surface.
U.S. Pat. No. 4,720,449 (POLAROID) discloses a thermal imaging method for producing colour images on a support carrying at least one layer of a colourless compound, such as di- or triarylmethane, by direct application of heat or by conversion of electromagnetic radiation into heat. The laser beam may have different wavelengths, typically in a range above 700 nm with at least about 60 nm apart so that each imaging layer may be exposed separately to convert a colourless triarylmethane compound into a coloured form, such as yellow, magenta, cyan or black, by controlling the focusing depth of the laser beam source to each colour forming layer. The colour forming compositions include di- or triarylmethane compounds, infrared absorbers, acidic substances and binders.
U.S. Pat. No. 4,663,518 (POLAROID) discloses a laser printing method for activating heat sensitive image forming dyes in three different layers on a support to provide an identification card containing a coloured pictorial image of the card holder, coloured text and machine readable digital code.
EP739748 A1(3M) discloses a laser-imageable direct write medium comprising an infrared dye in photoreactive association with a leuco dye, in which irradiation of the medium at a wavelength absorbed by said infrared dye converts said leuco dye to the corresponding dye. In example 6, such a leuco dye forms a cyan dye colour in combination with a black colour derived from the reduction of silver behenate.
U.S. Pat. No. 6,017,972 (M A HANNACOLOR) discloses the controlled color laser marking of plastics using pulsed and continuous wave lasers and Q-switched lasers.
US 2010099556 (ZINK IMAGING) discloses that the time and temperature of heating of color-forming layers can be controlled by using a laser in a pulsed mode with appropriate choices of laser power and pulse length.
JP 61-175077 and the corresponding U.S. Pat. No. 4,742,042 (MITSUBISHI PAPER MILLS) discloses a thermosensitive recording material containing a colorless dye precursor and a developer which, upon being heated, reacts with said dye precursor to develop color, wherein said thermosensitive recording layer contains as sensitizer a compound selected from a group consisting of p-benzylbiphenyl, diaryl esters of adipic acid, and the developer is bis(3-allyl-4-hydroxyphenyl)sulfone.
In conventional printing techniques, such as offset and inkjet printing, four colours (CMYK) are normally used to obtain optimal image quality and colour gamut. The colour laser marking systems described for producing security documents generally use three colours: cyan, magenta and yellow (CMY). The black colour (K) produced by colour addition of the three other colours (CMY) tends to be a brownish black colour rather than the desired neutral black colour. Adding a fourth layer to produce a neutral black colour makes the recording material more complex and expensive and would require an extra laser, making also the apparatus for producing the security card more complex and expensive. If a white background for the security document is desired, then usually three spectrally separated infrared absorbers are selected to produce a CMY-image. Adding a fourth black colour forming layer would make it very difficult to have four infrared absorbers with minimal overlap in their absorption spectrum and thus minimal colour contamination.
Therefore, it would be desirable to have a secure colour laser marking system for producing security documents with improved image quality (neutral black colour) without increasing the complexity of the laser marking apparatus or the recording material.